Explosive actuator



J. W- BLAIN ETAL March 19, 1968 EXPLOS IVE ACTUATOR Filed March 1, 1965 INVENTORS JIM W. BLAI N BY AUDLEY B. LEAMAN Agent I llnited States Patent ()fiice 3,3 73,686 Patented Mar. 19, 1968 Calif.

Filed Mar. 1, 1965, Ser. No. 436,188 Claims. (Cl. 102-22) This invention relates to an explosive actuator configuration capable of totally confining the products of explosion and more particularly to such a configuration having as its working member a radially expandable con tinuous sheath encapsulating an explosive core.

Because of the inherent simplicity, high reliability and relatively low input power requirements, the use of conventional explosive charges such as pentaerythritol tetranitrate (PETN), cyclotrimethlenetrinitramine (RDX), Primacord and mild detonating fuze (MDF) have become quite extensive in many types of ordnance devices as well as in missile and satellite separation systems.

Such explosives in general consist of an outer sheath and an explosive core coaxially located within the sheath. The explosives are susceptible to being classified into two groups. In the first group of explosives, for example, Primacord, MDF and the like, upon detonation of the explosive core the outer sheath completely disintegrates thereby contaminating the immediate vicinity with the byproducts of explosion and the sheath material. In the second group of explosives radial expansion of the outer sheath is purposely constrained and the sheath is merely used to transmit the initiation stimulus from one point to another without damage to any adjacent structure. Illustrative of the second group of explosives are US. Patents 2,982,210 and 3,027,839. Patent 2,982,210 discloses an explosive core encased within a reinforced metal sheath, the sheath being incapable of lateral transmission of the detonation stimulus. Patent 3,027,839 discloses a configuration having a strong outer tube, a relatively weak inner tube and a layer of black powder between the tubes. The stress to which the outer wall is subjected is reduced by the collapsev of the inner wall which provides more volume for the generated gases of explosion thereby precluding the expansion and rupture of the outer tube. Even in this type of configuration, however, the by-products of explosion inherently escape from the end of the tube and contaminate the surrounding vicinity.

In summation, while present uses of explosive charges have many advantages, they nevertheless suffer from a number of deficiencies which have not yet been successfully overcome. In particular, the problem of providing a hermetically sealed explosive actuation configuration using conventional explosives for use in areas where contamination of the surrounding environment by the explosive by-products is undesirable has not been successfully resolved.

In accordance with the invention, there is described an explosive actuator configuration which offers the advantages of using convetnional explosives and yet provides a hermetically sealed configuration which permits usage in areas previously considered not feasible for such explosives. More particularly, the advantages of the invention are realized by making the sheath surrounding the explosive core an active member in transmitting the force of explosion and thereby doing work on an adjoining body rather than as in the prior art a passive member that is either destroyed during detonation or merely is utilized to transmit the detonation stimulus axially along the line of the explosive core.

In essence, the explosive actuator configuration of the invention comprises an explosive core, the particular type of explosive utilized having been determined not to be critical and a radially expandable sheath surrounding the core, the sheath having a minimum ultimate tensile strength sufilcient to prevent rupture thereof when radially expanded by force of detonation of the explosive core. By means of this configuration the force of detonation of the core is transmitted to the sheath, the radial expansion of which does work on an adjoining body. Since the sheath remains continuous, that is, does not rupture, when radially expanded, the products of detonation of the core are confined within the sheath, thereby preventing contamination of the surrounding area.

A more complete understanding of the invention is facilitated by reference to the drawing in which:

FIG. 1 is a top view of one embodiment of an explosive system utilizing an explosive actuator configuration of the invention;

FIG. 2 is a side view, partially in section, of FIG. 1 taken along axis C-C of FIG. 1; and

FIG. 3 is a side elevational view of FIG. 1 taken along axis A-A of FIG. 1.

Referring more particularly to FIG. 1 the therein depicted illustrative explosive system comprises a conventional explosive initiator assembly 1 connected to an illustrative explosive actuator configuration 2 of the invention. The actuator configuration 2 is mounted in a fixed relationship to body 3 by screws 14 against which the actuator configuration is designed to do work. In this embodiment of FIG. 1 body 3 is a separation joint. In operation initiator assembly 1 produces sufiicient stimulus to detonate the explosive core of actuator configuration 2. The detonation shock front travels down the length of the activator configuration and is sufiicient to effect an immediate uniform increase in the diameter of the sheath surrounding the explosive core of the actuator configuration. The force generated by the radial ex ansion of the sheath is sufiicient to do work such as in the embodiment shown in FIG. 1 producing separation of body 3 at notch 4 in body 3.

The explosive system of FIG. 1 is shown in more detail in FIGS. 2 and 3 wherein like numerals designate like members. With reference to FIGS. 2 and 3, initiator assembly 1 comprises a housing 5 containing one or more detonators 6 in close proximity to the explosive core 7 of actuator configuration 2 to effect reliable initiation thereof. Housing 5 illustratively may be cast from a plastic such as polyurethane and polyvinylchloride. The detonator wires 8 are attached to detonator 6 and terminate to a conventional electrical connector 9 mounted on housing 5. Initiator assembly 1 is readily attached to explosive actuator 2 by a variety of methods well known in the art. For example, housing 5 may be either bonded with a suitable cement such as polyurethane or mechanically coupled to sheath 10 of explosive actuator 2. The initiator assembly 1 may provide either total hermetic type confinement or a semi-confined condition whereby the explosive by-products are deliberately permitted to vent from the initiator housing 5 to the outside atmosphere. In the latter instance and where desirable one or more vent holes 11 are located opposite the detonator 6. It is understood that the method of initiation of explosive core 7 of explosive actuator 2 depicted in the drawing is only representative of a variety of conventional initiation methods.

Detonation of explosive core 7 of explosive actuator 2 produces a shock front which travels down explosive actuator 2 to body 3. The travelling shock wave and associated pressure increase also causes an immediate uniform radial expansion of sheath 10 surrounding explosive core 7 of explosive actuator 2. The force generated by the radial expansion of sheath 10 is sufiicient to do work upon body 3. For the illustrative embodiment depicted in the drawing this work produces a separation of body 3 at notch 4 due to stress concentration. Upon separation of body 3 all products of detonation of explosive core 7 are trapped within sheath 10 and initiator assembly 1. Alternatively, as previously discussed, gases from the products of explosion may be optionally bled off through vent holes 11. However, for many contemplated uses of the explosive system, vent holes 11 would be eliminated from initiator assembly 1 since contamination of the surrounding atmosphere cannot be tolerated.

As depicted in FIG. 2, the separation joint 3 comprises explosive actuator 2 held against joint 3 by retaining backup members 12 and 13 which by means of screws 14 and members 15 are, in turn, affixed to joint 3. Commensurate with the art the trapezoidal cross section of explosive actuator 2 is chosen as an optimum shape which facilitates expansion of sheath 10 and directs the maximum detonation force of core 7 towards joint 3. Notch 4, although not necessary, facilitates uniform rupture of body 3 by the radial expansion of sheath 10.

The explosive actuator configuration depicted in FIG. 2 of the drawing is readily fabricated by either casting or extruding a radially elongatable sheath material 10 over a conventional explosive core 7. The explosive activator cross section may be fabricated to assume any shape desired to fit a particular situation. Round, square, half round, trapezoid shapes, among others, are typical cross sections. Commensurate with the art, the trapezoidal shape appears the most eificient due to the tapered sides which are self-relieving during expansion. The choice of the particular shape to be utilized is considered in the skill of the art. The type of explosive used for the core material 7 is not critical and, for example, may consist of RDX, PETN, MDF, FLSC, lead azide, nitrocellulose propellent and lead styphnate, among others. The particular choice of explosive for the core is within the skill of the art and is dependent upon the intended use of the explosive actuator.

The choice of material for the sheath 10 surrounding the core 7 is dependent upon the type of explosive utilized as the core material. In order to successfully function as an active member of the explosive actuator, the sheath material must exhibit sufiicient radial elongation in order to perform useful work against an adjoining body, and further have a sufiicient minimum ultimate tensile strength to prevent rupturing of the sheath material when radially elongated by the generated force of detonation of the explosive core. It can be appreciated that the generated force of detonation and gas pressure of the core and the characteristics of the sheath material surrounding the core are interdependent with smaller detonation forces requiring smaller ultimate tensile strengths and larger detonation forces requiring larger ultimate tensile strengths to preclude rupture of the sheath and to ensure the total confining of the products of detonation within the sheath. The following Table l is illustrative of the dependency of the sheath characteristics on the explosive force of detonation. In this table there is set forth the required minimum, preferred and optimum sheath characteristics for an explosive having a detonation rate of 7000 meters per second. As is understood by the art, explosive compositions are typically characterized in terms of detonation rates. These values are found, for example, in military handbooks such as TM9-l910, page 324. The characteristics in following Table I are based on ASTM specifications.

p.s.1.). (4) Shear Die 0 )lbs.

per inch).

80 At least 200 At least 375.

Based upon the preceding discussions, it is considered Within the skill of the art to determine equivalent characteristics when smaller or larger detonation forces are utilized. Detonation rates smaller than 7000 meters per second will naturally permit the utilization of a sheath material having smaller characteristics than those set forth in Table I. Detonation rates greater than 7,000 meters per second will, of necessity, dictate the use of a sheath material having larger characteristics in order to prevent rupture of the sheath. Naturally the characteristics set forth in Table I are dependent not only on the particular choice of sheath material utilized but the thickness of the sheath material. In general increasing the thickness increases the characteristics set forth in Table I. Accordingly, the particular choice of material and the thickness utilized are discretionary to the art provided that the resulting combination of material and thickness thereof exhibits characteristics equivalent to those set forth in Table I for a detonation rate of 7,000 meters per second. It is to be appreciated that the invention is not limited to a sheath formed of a single material. Sheaths formed of two or more differing materials, for example, a thin wall stainless tube countering an inner plastic tube are within the scope of the invention provided the resulting composite sheath body exhibits characteristics in accordance with Table I.

Naturally, any radial elongation of the sheath against an adjoining body will perform work. However, useful work is generally considered to be performed when the radial elongation of the sheath is in the order of .50 percent. Materials exhibiting a greater elongation characteristic, desirably at least 250 percent and preferably at least 500 percent, enhance the work performed by the sheath against an adjoining body.

Operation With reference to the drawing, in one illustrative embodiment of the invention initiator assembly 1 was cast from polyurethane. One conventional detonator 6 designated S142, made by Universal Match Company, was encapsulated in the assembly in a logical position to assume initiation of explosive core 7. Termination of the explosive actuator 2 to the initiator assembly housing 5 was provided for by two core holes in the polyurethane block which received the ends of the explosive actuator 2. The explosive actuator was bonded into position in initiator assembly housing 5 with polyurethane, designated PR1535 made by Products Research, to give a hermetic seal. Body 3 was a 4-inch by 6-inch test joint, with a A;- inch thick HM21AT8 magnesium target under the separation notch 4. Explosive actuator 2 consisted of a 5- grain MDF explosive core 7 having a detonation rate of 7000 meters per second encapsulated in a one-half inch thick sheath of polyurethane. The diameter of core 7 was 0.050 inch.

Initiation of detonator 6 produced sufficient stimulus to initiate the MDF explosive core 7 within sheath 10. The detonation shock front travelled down the length of the explosive activator at approximately 7000 meters per second. The combined effects of shock, gas pressure and transfer of momentum of the MDF lead particles to the sheath material was sufiicient to effect an immediate radial elongation of the sheath of approximately 250% within 40 microseconds, the elongation proceeded radially at approximately 800 feet per second. This elongation caused the sheath to expand from an initial 0.50 inch outside diameter to an approximately 1.25-inch diameter at its maximum elongation. The force generated by the sheath elongation was sufficient to produce separation of body 3 at notch 4. Upon separation all MDF particles and gases were trapped within the sheath 10 and initiator assembly 1 without rupture of either.

While certain preferred embodiments of the invention have been specifically disclosed herein, it is understood that the invention is not so limited. Many variations will be apparent to those skilled in the art and the invention is to be given the broadcast interpretation within the scope of the appended claims.

What is claimed is:

1. An explosive system comprising an explosive actuator having an explosive core and a radially expandable sheath surrounding said core, means adapted to detonate said core, and a body adjacent to said explosive actuator adapted to be acted upon and engaged by said sheath when said sheath is radially expanded by detonation of said core, said sheath remaining continuous when radially expanded.

2. An explosive system in accordance with claim 1 wherein said sheath has a minimum ultimate elongation of 50 percent.

3. An explosive system in accordance with claim 2 wherein said sheath exhibits a minimum ultimate tensile strength characteristic equivalent to 600 psi. for a detonation rate of said core of 7000 meters per second.

4. An explosive system in accordance with claim 3 wherein said sheath exhibits a minimum hardness characteristics equivalent to 30A Shore for a detonation rate of said core of 7000 meters per second.

5. An explosive system in accordance with claim 4 wherein said sheath exhibits a minimum 300 percent modulus equivalent to 250 p.s.i. for a detonation rate of said core of 7000 meters per second.

6. An explosive system in accordance with claim 5 wherein said sheath exhibits a minimum shear die C equivalent to 80 pounds per inch for a detonation rate of said core of 7000 meters per second.

7. A method for explosively activating a body comprising forming an explosive activator having an explosive core, a radially expandable sheath surrounding said core and means adapted to detonate said core, said sheath remaining continuous when radially expanded by the generated force of detonation of said core, positioning a body 5 adjacent to said explosive activator and detonating said core, the resulting radial expansion of said sheath causing said sheath to act on said body.

8. A method in accordance with claim 7 wherein said sheath exhibits a minimum ultimate tensile strength char- 10 acteristic equivalent to 600 p.s.i. for a detonation rate of said core of 7000 meters per second.

9. A method in accordance with claim 8 wherein said sheath exhibits characteristics equivalent to a minimum hardness of 30A Shore, a minimum 300 percent modulus of 250 psi. and a minimum shear die C of 80 pounds per inch for a detonation rate of said core of 7000 meters per second.

10. A method in accordance wtih claim 9 wherein said sheath has a minimum ultimate elongation of 50 percent.

References Cited UNITED STATES PATENTS 2,891,475 6/1959 Dolan et a1 102-27 3,027,839 4/1962 Grandy et al. 102-27 3,106,131 10/1963 Barr et al. 10239 X 3,207,073 9/1965 Miller 10227 3,032,356 5/1962 Botsford 10249 3,129,663 4/1964 Schnepfe 102-27 BENJAMIN A. BORCHELT, Primary Examiner.

R. V. LOTTMANN, V. R. PENDEGRASS,

Assistant Examiners. 

1. AN EXPLOSIVE SYSTEM COMPRISING AN EXPLOSIVE ACTUATOR HAVING AN EXPLOSIVE CORE AND A RADIALLY EXPANDABLE SHEATH SURROUNDING SAID CORE, MEANS ADAPTED TO DETONATE SAID CORE, AND A BODY ADJACENT TO SAID EXPLOSIVE ACTUATOR ADAPTED TO BE ACTED UPON AND ENGAGED BY SAID SHEATH WHEN SAID SHEATH IS RADIALLY EXPANDED BY DETONATION OF SAID CORE, SAID SHEATH REMAINING CONTINUOUS WHEN RADIALLY EXPANDED. 